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Level Detector
The level detection circuits contain a wide dynamic full-wave rectifier, logging circuit and a unipolar drive amplifier. These circuits will accurately detect the input signal level over a 100dB range from 30nA to 3mA peak-to-peak.

Referring to the block diagram of the level detector, the REC-IN input is an AC virtual ground. When applying signals a DC blocking capacitor (C-IN) is used since REC-IN has a DC potential of about 2.1V above ground. The value of the input resistor is set to give a +/- 1.5mA peak signal. For +/- 15V operation this corresponds to 10K.

The full wave rectifier is followed by a logging diode whose pair transistor has a fixed collector current set by R-REF. For 15V operation R-REF is 1M5. The signal on the LOG-AV output is the log of the average of the absolute value of the input current. The value of C-AV affects the attack and decay times of the circuit.

The LOG-AV signal is buffered by a unipolar amplifier stage. RL provides an emitter load for the output transistor while the 39K/1K feedback resistors give a gain of 40.

Referring back to the main circuit diagram, the relation to the components shown on the block diagram can be clearly seen. U2:B is the level detector section if the SSM2120. The value selected for C-AV (C2) is 2.2uF, which was found by experimentation to give the best overall performance on a wide range of music. The level detector was found to respond to lower frequencies rather more than higher frequencies, so the input resistor (R17) was shunted by a high-pass filter (C7 and R18) which begins to take effect above about 1.5KHz. The values of these components were also established by experimentation and listening tests.

The level detector output passes to the dynamic range control (RV1) via a filter circuit (R6 and C3). This filter smooths the sharp attack edges of the signal to reduce the audible effects when a high level of expansion is used.

SW1, and SW3 on the right channel, are contained in one double pole component. When the switch is open, each channel operates independently, and when it is closed the two channels are controlled together. D1 and D2 ensure that the higher level detector output at that instant reaches the base of Q1. The transistor is configured as an emitter follower, and it's base-emitter voltage drop compensates for the voltage drop in the diodes (to within 100mV). The final control signal is buffered by U3:A.

Audio Signal Path
The audio signal into the unit is buffered by U1:A. This is preceded by a DC blocking capacitor (C1) to remove any DC offset on the input. The value of C1 together with R1 is set to give a low frequency roll-off, with a -3dB point at about 30Hz. Allowing lower frequency signals through at a high level can cause an unpleasant pumping effect on signals with a high bass content.

The input to the voltage controlled amplifier (VCA) section of the SSM2120 (U2:A) is a virtual earth. The audio voltage signal is converted to a current by R15, while R11 and C4 ensure stability. The current output from U2:A is converted back to a voltage signal by U1:B. C5 ensures stability, while C6 is a DC blocking component.

The current into the CFT input of U2:B is adjusted by RV2 for minimum distortion. This operates by trimming out internal voltage offsets, and the setting method is described later.

The VCA has two control inputs. Increasing the voltage on the + control input causes an increase in gain, while increasing the voltage on the - control input decreases the gain. SW2 (ganged with SW4 on right channel) selects the appropriate input depending upon whether expansion or compression is required. On the prototype a three position rotary switch was used with the centre position connected to neither input to give a "flat" setting. Both control inputs must be connected to ground by resistors not exceeding 200R. R16 sets the amount of compression or expansion obtained with RV1 at maximum, the value of 2K2 giving about 3:1 which in practice is ample.

In a fixed range system R16 would be connected directly back to the output of the level detector (pin 3 of U2:B), the value being selected to give the desired level of compression or expansion. The additional components are necessary to allow a smooth manual adjustment of the compression or expansion level.